Certain embodiments of the present invention relate to providing access to broadband communication systems. More specifically, certain embodiments relate to an apparatus which provides an interface to connect to broadband synchronous optical networks in order to provide a variety of synchronous and packet network connections.
In the past, a variety of transmission technologies have been used to electronically transfer large amounts of digital information, including both terrestrial and satellite links. Terrestrial facilities that have commonly been used include both buried and above-ground cable, microwave radio and most recently, optical fiber, which offers the largest bandwidth. Networks used for such high capacity data transport systems are typically synchronous networks.
A synchronous network is one example of what is traditionally referred to as a circuit-switched network. In a synchronous network, data is transmitted from one location to another as a continuous stream of digital information moving from the source to the destination at a constant rate. The stream is organized as a sequence of frames, each frame containing a fixed number of fields in a defined order, each field of the same length. An end-to-end connection or “circuit” in a synchronous network exists as a collection of individual segments which are assigned when the circuit is built or “provisioned”. At the time that a circuit is provisioned it is assigned the use of one or more of the fields in the frames exchanged across a given segment, and a circuit may be assigned a different field within the frames carried on different segments. The transfer of data at the point of connection of one segment to another is time synchronized, and does not add significant delay. Because the data on any segment moves at a constant rate, and no delay occurs at the connections between segments, the time needed to travel from one end to the other end of a circuit is fixed. The Synchronous Optical Network (SONET) and Synchronous Digital Hierarchy (SDH) are the principal synchronous optical network standards currently in use. In the SONET standard, the term “circuit” in the above discussion corresponds to the SONET term “path”, and the term “segment” corresponds to the SONET term “link”. An example of a path in a SONET network is shown in FIG. 1.
In most cases, no single user needs all of the capacity of an optical fiber-based transmission system, so the standards have been designed to provide a means to share the bandwidth. For example, SONET networks typically operate at data rates of between 51.84 megabits per second (Mbps) and 10 gigabits per second (Gbps). Within that range, a device called an add/drop multiplexer (ADM) can be used to insert or extract a lower bit rate stream to or from one of a higher bit rate. A diagram showing the SONET hierarchy and the relationships between bits rates is illustrated in FIG. 2.
In contrast to circuit switched networks, packet networks consist of a mesh of nodes interconnected by links, and data is exchanged in bursts called packets. The use of packet networks is growing in popularity due to the flexibility offered by the ability of a packet network to efficiently handle multiple data streams of widely varying bandwidth. This flexibility is one of the factors helping to bring about a convergence of data and voice networks. The packet contents include the address of its destination, and it is the function of each node to direct each packet that it receives to a link that will send it closer to its destination. In general, a packet is queued at a node before being forwarded to the next node in the path, because it may have to wait for the outgoing link to become available. Packets may contain voice, data, or video information, and can be of varying length. The amount of time that a packet takes to travel from the source to the destination varies based upon a number of factors including the number of nodes, the speed of the links, and the queuing delay that occurred at each node. Each of the services supported on a packet network has its own set of requirements including, for example, end-to-end delay, packet loss, and privacy. Designers of packet networks take those requirements into consideration.
Synchronous optical networks are the primary transport mechanism for long distance transmission of information, and are becoming increasingly important in metropolitan areas. At the same time, the use of packet networks is growing rapidly due to their ability to efficiently carry multiple data streams of widely varying bandwidth. With the passage of time, the number and variety of data services, the number of users, and the total bandwidth required at any particular user location will grow. Some legacy equipment requires lower speed synchronous network connections, while other equipment requires a packet network interface. In some applications, more than one synchronous optical link may be needed to support the total bandwidth required. As user demand for higher bandwidth connections grows and synchronous optical networks expand, support for connections of varying bandwidth will become increasingly important. The result is an ever-growing need for high-capacity, highly-functional, cost-effective systems for the connection of synchronous optical networks to packet networks and to lower speed synchronous networks.
The functionality that may be needed to connect a SONET or SDH synchronous optical network and a packet network includes that of an Add-Drop Multiplexer (ADM) or terminal, a Digital Cross-Connect (DCC), and a Multi-Service Provisioning Platform (MSPP). ADMs are used to transport SONET or SDH traffic on network ring topologies. An example of such a SONET ring is shown in FIG. 3. The most popular of these ring topologies are Unidirectional Protected Switched Rings (UPSR) and Bidirectional Line Switched Rings (BLSR). In this arrangement, the nodes on the ring are linked by two optical fiber connections that transmit data in opposite directions. Should one of the optical fibers experience a failure, the nodes in the ring are still able to communicate using the other optical fiber. The ADMs are nodes on such rings that are used to arbitrate (add or drop) traffic to or from the ring. Rings are interconnected by gateways, as illustrated in FIG. 4. The client traffic on the ADM (the traffic that is added or dropped from the network ring) is normally transmitted at a lesser data rate than the network traffic (the traffic on the ring). Typical ring traffic rates for both SONET and SDH are 155 Mbps, 622 Mbps, 2488 Mbps and 9953 Mbps. These correspond to OC-3, OC-12, OC-48 and OC-192 rates for SONET respectively, and to STM-1, STM-4, STM-16 and STM-64 rates for SDH respectively. Client traffic on the ADM can either be a lower SONET or SDH rate than the ring rate, or it can be a PDH rate (Plesiochronous Digital Hierarchy), such as DS1, DS2 or DS3 or E1, E2 and E3. The DS1 rate is 1.544 Mbps, DS2 is 6.312 Mbps, DS3 is 44.736 Mbps, E1 is 2.048 Mbps, E2 is 8.448 Mbps, and E3 is 34.368 Mbps.
A SONET/SDH terminal performs a function similar to that of an ADM except that the network connection is not in a ring configuration. A terminal terminates a high speed point-to-point SONET path, and hands off a number of lower rate lines and paths on the client side. For example, an OC-3 terminal could be used to terminate an OC-3 path and hand off three DS3 lines on the client side.
The DCCs are used to switch and groom traffic between different lines and paths. A network may include several ADMs and terminals to arbitrate or terminate traffic along rings or point-to-point connections, and a DCC will be used to switch the traffic between all the paths. A DCC is a circuit switch, which means that all connections are provisioned statically.
The Multi-Service Provisioning platform combines the function of the DCC, the ADM, and the terminal along with the ability to support data protocols such as Ethernet to the client users. In all instances today, these MSPPs are scalable platforms based on a chassis. This means that to build a useful system, a user needs to install a specific circuit card supporting each function. The purpose of the chassis is to hold the required circuit cards, provide an electrical interconnect or “backplane” to connect signals from one card to another, and to supply power for system operation. For example, separate cards are needed for switching, supporting the ADM function, supporting and mapping DS1 traffic, supporting and mapping DS3 traffic, and supporting and mapping Ethernet traffic. FIG. 5 illustrates an example chassis arrangement of an MSPP 500, showing Ethernet interface card 502, cross-point switch card 504, synchronous optical interface cards 506 and 508, and processor card 510. The silicon devices developed to support these platforms tend to implement an ever increasing but still small portion of the needed functionality. For instance, there are devices on the market supporting SONET framing, DS1 framing and mapping, DS3 framing and mapping, DS1 mapping into DS3 (known as M13 mappers), Ethernet-over-SONET mappers, and digital cross-connects. Building a system is complex and costly due to the number of cards and/or individual integrated circuit devices required. The variety and number of network connections that can be supported by the MSPP system is limited by several factors, including the level of functionality and number of connections on each integrated circuit device, the number of circuit cards that can be contained within the chassis, and the number of signals that must be carried by the backplane.
As can be seen from the above discussion, there is a fundamental disconnect between the packet network environment and core optical networks such as SONET and SDH. The relatively high cost of the technology typically used to fill this gap hinders network growth and further expansion of support for metropolitan optical networks. Accordingly there is a need for a more compact, cost-effective, and more flexible solution to providing packet network and time-division-multiplexed type services over SONET and SDH-based optical networks.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.